serve s3: fix error handling for listing non-existent prefix - fixes #7455

Before this change serve s3 would return NoSuchKey errors when a non
existent prefix was listed.

This change fixes it to return an empty list like AWS does.

This was discovered by the full integration tests.

cmd/serve/s3/backend.go

@@ -84,7 +84,10 @@ func (b *s3Backend) ListBucket(bucket string, prefix *gofakes3.Prefix, page gofa
 		err = b.entryListR(bucket, path, remaining, prefix.HasDelimiter, response)
 	}
 
-	if err != nil {
+	if err == gofakes3.ErrNoSuchKey {
+		// AWS just returns an empty list
+		response = gofakes3.NewObjectList()
+	} else if err != nil {
 		return nil, err
 	}